\textbf{185-} Using a wire of length $100$ m, we form a circular loop of radius $R$. How many centimeters should $R$ be so that if current $I = 10\,\text{A}$ passes through the loop, the magnetic field at its center is $T = 2.5 \times 10^{-2}\,\text{T}$?
$$\left(\mu_\circ = 4\pi \times 10^{-7}\,\frac{\text{T.m}}{\text{A}}\right)$$
\begin{itemize}
\item[(1)] $20$
\item[(2)] $20\sqrt{2}$
\item[(3)] $40$
\item[(4)] $40\sqrt{2}$
\end{itemize}